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Exhaust differences 3.5 -> 3.9 EFi


FridgeFreezer

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Thanks to Moglite I have acquired a rather nice SS system for a 3.9 Range Rover, which is destined to be fitted to an '86 3.5 EFi.

He's also given me the 3.9 manifolds, but I'd like to avoid having to fit them if at all possible.

So, will they bolt straight up or do I have to skin some knuckles? <_<

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Thanks to Moglite I have acquired a rather nice SS system for a 3.9 Range Rover, which is destined to be fitted to an '86 3.5 EFi.

He's also given me the 3.9 manifolds, but I'd like to avoid having to fit them if at all possible.

So, will they bolt straight up or do I have to skin some knuckles? <_<

Well if there's any difference between the manifolds on the 3.5 in my truck and the old 3.9 in my garage I can't spot it. Certainly nothing you need to worry about.

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That seems to be a widely held view, yes... :lol:

I am not sure about that. I have spoken to a few companies that do "high performance" exhaust systems, JE to name but one, and the general view seems to be that the cast efi manifolds, siamesed down pipe type, pre efi are round, are quite good flow and that there is little to be gained from a standard engine putting on tubular manifolds as the head valve arrangement is the limiting factor.

I do suspect that by "little to be gained" they actually mean that it's not cost effective to make properly balanced length tubular manifolds in the commercial arena.

Both myself (Falcon) and a friend (Rimmers) have tubular manifolds & systems, both of us noticed gains at the top end but difficult to quantify. Rimmers sounds better than mine burbling along but when I hit the revs you can hear it howl where as the Rimmers just sounds like a louder burble.

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I am not sure about that. I have spoken to a few companies that do "high performance" exhaust systems, JE to name but one, and the general view seems to be that the cast efi manifolds, siamesed down pipe type, pre efi are round, are quite good flow and that there is little to be gained from a standard engine putting on tubular manifolds as the head valve arrangement is the limiting factor.

I do suspect that by "little to be gained" they actually mean that it's not cost effective to make properly balanced length tubular manifolds in the commercial arena.

Both myself (Falcon) and a friend (Rimmers) have tubular manifolds & systems, both of us noticed gains at the top end but difficult to quantify. Rimmers sounds better than mine burbling along but when I hit the revs you can hear it howl where as the Rimmers just sounds like a louder burble.

There's also the issue of heat transfer within the engine compartment, the cast manifold help keep the heat in better so that it can go down the pipes and be lost elsewhere, while the tubular pipes emit the heat a lot easier and you'll then start getting heating issues around the engine compartment unless you start wrapping the exhausts with the ol' wrapping tape stuff.

Also the cast one's are tuned correctly for the firing sequence with one being shaped for the firing sequence on that side, and the other one tuned for the other, most tubular manifolds being 8 into 2 into 1 (I'll call it 8:2:1 from this point) are actually a poor tuned-length for a low revving V8, the 8:2:1 meaning that the gases meeting at the same point on either side of the block is actually a tuned length for high-revving engines where top-end power is a requirement. While the 8:4:2:1 (or looking at one side, 4:2:1 before it meets the Y pipe) is actually correct for a low revving engine that requires a good spread of torque and not worrying about 10 or so less BHP at the top of the rev range.

And then there's the thermal expansion issues, if you look at the 4.6 stainless steel headers your notice a contraption between the pipes on one of them (or was it both.....), this is due to the stainless when heated expanding at a different and quite noticeable rate than the alloy block and headers. This little contraption is there to act as a expansion-joint so that the header can be stretched and squeezed by the block during heating and cooling. This doesn't tend to happen with the cast headers as their thermal expansion co-efficient is closer to that of the alloy.

Otherall, if I didn't need to cut my chassis about or spend hours trying to find a cast header set-up that would fit my 109, I would have used them :rolleyes: but the tubular fits in my engine bay so I got to now wrap them in heatwrap tape :(

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Yes that's why there is almost as much fibreglass tape around mine as there is tubing, it also helps with thermal extraction effect. Mind you heat build up in the engien bay is not normally a problem in Scotland :rolleyes: just go faster :D

The equal length header pipes do tend to benefit the top end as you say, which is probably why mine howls as opposed to my mates Rimmer kit, though I can't remember if it is equal length or just 'convenient' length. So mine are basically 4:1 with 2:1 at the Y

My header flanges are all separated for that very reason but small inconsistences in manufacture due to welding means they can be a pig to line up.

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Hmmm, all was going well until time came to bolt the crossmember back in (it'd been down for gearbox work) and then it suddenly didn't fit anymore :(

Vehicle is an '86 3.5 EFi Classic, zorst is off a J-reg 3.9 Classic - did they change the crossmember shape / position with the later classics?

And do I ring paddocks and order a new crossmember, or a new exhaust? :lol:

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As Deano says. Alternatives are:-

1. Get a tubular cross member, bolt pattern is the same. Might be able to pick one up from a breaker.

2. Modify the box section one by cutting out slots then plate over, can be done.

3. Leave it out altogether, I know somone whole drove around for a long time without one, including moderate off roading without any ill effects that we know of, that was an 85.

Coice is yours really. :)

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Yep I have an '87 that someone didn't bother to replace the x-member. Mind you, you can see kinks in the chassis where it should be,so I gues it's there for a reason.

They can corrode between the flange and the chassis, should have mentioned that :rolleyes: , the CSK had finger sized holes and the remaining plate was rippled when I took it off.

I don't believe it will add anything to the bending stiffness front to back and any benefit to twisting between front & rear axles must be pretty marginal

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  • 2 weeks later...
Both myself (Falcon) and a friend (Rimmers) have tubular manifolds & systems

So, which one do you get if you don't really want tubular manifolds, haven't got a massive budget but want a nice noise coming out of the back then?

I was tempted to buy a boy-racer Peco BB4 pipe and fab pipework to suit, but the mountings were impossible as the Peco was too long.

Ideas Niall? :unsure:

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a couple of very simple options for more noise:

- cut the rear box off and replace with straight pipe.

- cut the middle box off and replace with straight pipe.

Fors and againsts those options:

-cut the rear box off and replace with straight pipe- Easier by far as everything is lined up and helps when diverting to side exit. Also you still have a restrictive baffle type front silencer which slows the exhaust gases down.

- cut the middle box off and replace with straight pipe- More difficult as the front silencer is not aligned due to different inlet and exit points so needs some extra welding. Unless you have a pick-up you really want to consider using a second single pipe rear box in the front position because the resonance inside the cabin is quite wearing on a long run especially for back seat passengers. The tailpipe is more vulnerable unless side exited. Performance should be better by a wee bit with that half ton of front box and restriction out the way.

I prefer the second option

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